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HOW-TO: Replacing M117 Coolant Thermostat (5.6-liter motors)

Since moving to Maryland in late October, I've noticed that with the cooler temps, it has been taking my 560SEC an extremely long time to reach operating temperature, as indicated on the coolant temperature gauge in the instrument cluster. Even after driving 10-20 miles, the coolant temp gauge was only registering 75-80C, not even quite getting up to the 80C line. This is in 40-degree ambient temperatures. There was plenty of hot coolant and the interior of the car was heated up as normal with ACC.

This reluctance to get up to a proper indicated 80-95C operating temperature (depending on load and ambient temperatures) is a classic sign of thermostat failure -- the thermostat is stuck in the "open" position.

Thus, I resolved to change the thermostat. First of all, I checked my records for the 560SEC. They showed that the thermostat had last been changed in October of 2009, at 180,000 miles -- that was 60,000 miles ago. It was using a Behr thermostat. So, with eight-plus years and 60,000+ miles on the thermostat, it was definitely time for a change.

I pulled a fresh, new Behr replacement thermostat from my parts stock, and went to work on the job. The replacement rubber o-ring is included in the Behr thermostat kit.

NOTE: Wahler also makes a replacement thermostat of good quality. However, the metal is "bonded" (pressed) together, whereas the Behr unit uses a slightly different design, that in my opinion, is a bit more reliable. I have heard a couple of experienced mechanics say that the Behr design has proven to be a bit more reliable in practice, because the bonding can be a weak spot.

Here is a visual comparison between the Wahler and Behr thermostats for the M117 and [early] M119:

The first step is to release the pressure at the coolant expansion tank, by loosening (twisting) the aluminum cap at the tank. ONLY DO THIS WITH A COLD ENGINE THAT HAS NOT BEEN DRIVEN IN AT LEAST THREE OR FOUR HOURS. You do not want to release the expansion tank cap when the engine/coolant are hot and/or under pressure. WAIT UNTIL THE CAR IS COLD.

Next up, put a proper drain pan under the radiator on the passenger side, and unscrew the light-blue "pet cock" (radiator drain plug) in the bottom of the radiator using a small Philips head screwdriver. Quickly place the pan under the end of the radiator to catch the draining coolant. You should see approximately 1.5-2 gallons of coolant drain from the radiator and expansion tank into the pan. Quite a lot -- a good opportunity to replace this drained coolant with new coolant, especially if your coolant is more than two years old.2017-12-23 17.30.49.jpg

Here is what the stubby and tiny screwdrivers I used look like. These are excellent for getting into the tight confines of the engine compartment and under the radiator of my lowered 560SEC. The small/tiny Philips screwdriver is great for loosening hose clamps in tiny spaces.2017-12-23 17.37.51.jpg2017-12-23 17.38.07.jpg

Next up, while the coolant is still draining from the radiator, you need to remove the lower radiator hose from the thermostat housing. Using your stubby/tiny Philips head screwdriver, loosen the ring clamp and carefully slide it down the hose and out of the way. Then using your hand, slowly work the end of the lower radiator hose back and forth and off of the thermostat housing flange.

Be sure to visually inspect the end and body of the lower radiator hose for damage, cracking, or other age and temperature related wear. If it is in bad condition, make a note that you need to replace it ASAP and order a new one. If it is in good condition, as my hose was (although it will need replacement in the coming years), then move it aside as the coolant drains from the top of the hose and the thermostat housing into your drain pan still under the car.

Moving the upper end of the hose out of the way after removing from the thermostat housing flange.2017-12-23 17.39.28.jpg

Next up, you need to prepare to remove the three 10mm bolts that hold the thermostat housing to the water pump. Two of them are easily reached with a 10mm socket on a 1/4" or 3/8" ratchet. The third bolt is behind and below, and takes more effort. It is best to use a deep 10mm socket for this, or your regular 10mm socket on a short 1/4" extension to reach behind and remove it. Eventually, you'll get it using a combination that you have to discover. You can see I tried a couple of combinations of extensions and flexies, but eventually settled on a 10mm deep socket on a 3/8" ratchet to get at the last/back bolt holding the thermostat housing to the water pump. It will continue to drain as you remove the three bolts.2017-12-23 17.40.05.jpg2017-12-23 17.40.27.jpg2017-12-23 17.44.32.jpg

Once the three bolts are removed, you need to CAREFULLY prise (pry) the thermostat housing off of the water pump. I did this with a skinny flat-bladed medium screwdriver, and a couple of light taps with a small ball-peen hammer. This separated the top of the thermostat housing away from the water pump enough to break the seal, and then I could remove it the rest of the way by working it back and forth with my hand.

Before replacing the thermostat into the water pump housing, go back underneath the car and re-insert the radiator drain pet-cock. The tightening torque for this pet-cock is only 1.5 Nm, so DO NOT overtighten it. 1.5 Nm is VERY VERY small, and basically you should stop tightening the pet-cock as soon as you begin feeling resistance.

IMPORTANT:MAKE SURE THAT THE LITTLE NIPPLE (AS SHOWN IN THE PHOTOS DIRECTLY ABOVE) IS PLACED AT THE TOP OF THE WATER PUMP HOUSING WHEN INSERTING THE THERMOSTAT INTO THE WATER PUMP. IT IS IMPERATIVE THAT THIS NIPPLE BE IN THE TOP POSITION.

Reinstallation of the thermostat is essentially the reverse of the removal. After placing the thermostat/o-ring (with the nipple in the top position) into the water pump, carefully replace the thermostat housing, and insert the three bolts. The tightening torque for these three bolts is only 10Nm -- this is not very much, so DO NOT overtighten them into the soft aluminum of the water pump housing.

After replacing and tightening the bolts, replace the lower radiator hose to the bottom of the thermostat housing. Tighten the ring clamp.

Next, you need to replace coolant. You should add coolant in two places: at the expansion tank (which should be totally drained), and at the upper hose of the radiator.

Continue filling the expansion tank until it is full up to the filling line. Replace the expansion tank cap (I replaced mine with a brand new MB unit, made by OEM Reutter). These are cheap and a good idea to replace every 10 years or so.

Next, remove the upper radiator hose where it attaches to the top of the water pump, by loosening the ring clamp. Using a long/skinny funnel, pour more coolant directly into this hose until it won't accept any further coolant. At this point, re-attach the hose back to the water pump.

Next, you want to start the engine, and let it warm up in the garage. You want to let it heat up to the point where the thermostat begins to open. Do this by starting the engine, and putting the heater (ACC) on "DEFROST" mode and turning the fan and temperature wheel all the way up.

While the car is warming up, checking the integrity of the things you disturbed during this repair: the expansion tank cap (make sure it's tight!), the hose connections for the upper and lower radiator hoses, and the place where the thermostat housing is bolted to the water pump housing. Check the radiator pet-cock drain, as well. You want to visually ensure that NONE of these areas are leaking coolant.

It's likely that your engine will have an air-bubble introduced into it with the draining and replacement of the coolant, so it may take a few minutes (perhaps even 10-15) for the coolant temperature gauge to come off of its peg. However, once the air bubble is burst, and the thermostat opens, you will see the needle come off of its lower peg and begin to show the correct coolant temperature of the 80-90C range.

Once it's warmed up (not HOT), and if you haven't run the engine too long to get things hot and pressurized, you can CAREFULLY remove the expansion tank cap (protect yourself with a shop rag, just in case) and check the coolant level in the tank. You will need to "top it off" with additional coolant to bring the level up to the appropriate place, as the thermostat opening likely caused the coolant level in the expansion tank to drop. Your coolant light on the dashboard may go on as well, indicating a low coolant condition.

Expect that you will need to add approximately 1.5-2 full gallons of coolant to the system. This is a majority of coolant capacity, so adding fresh coolant is a good idea and not re-using the old coolant, particularly if the coolant is more than 1-2 years old.

With that, you're pretty much done. Dispose of the old coolant as appropriate and in accordance with your local chemical disposal laws.

Re: HOW-TO: Replacing M117 Coolant Thermostat (5.6-liter motors)

I completed this HOW-TO, for all who are interested in replacing their M117 thermostat. Note that the job will require approximately 1-1.5 hours for the person who has not done it before, and who has appropriate tools.

The Following User Says Thank You to gerryvz For This Useful Post:

Re: HOW-TO: Replacing M117 Coolant Thermostat (5.6-liter motors)

Gerry - I don't get to this site too often - but I've got to say that even though it's primarily a 500E site, the "how-to's" you post in the 126 section are really great. Thanks for sharing your wealth of knowledge and experience.